There is a constant increase in the demand for real-time particle control. Especially the real-time exhaust control of combustion engines, such as vehicles, requires reliable and non-expensive particle monitoring. Requirement for particle control exists also e.g. in indoor air quality and outdoor monitoring or with air traffic safety. The particle amount is in most cases expressed as particle mass concentration, in mg/m3 or equivalent.
Various particle measurement devices are based on electrically charging particles and measuring the electrical current carried by such charged particles. One such prior art method and apparatus for measuring fine particles is described in document WO2009109688 A1, Pegasor Oy, Nov. 11, 2009. In this prior art method clean, essentially particle free, gas is supplied into the apparatus and directed as a main flow via an inlet chamber to an ejector provided inside the apparatus. The clean gas is further ionized before and during supplying it into the inlet chamber. The ionized clean gas may be preferably fed to the ejector at a sonic or close to sonic speed. The ionizing of the clean gas may be carried out for example using a corona charger. The inlet chamber is further provided with a sample inlet arranged in fluid communication with a channel or a space comprising aerosol having fine particles. The clean gas flow and the ejector together cause suction to the sample inlet such that a sample aerosol flow is formed from the duct or the space to the inlet chamber. The sample aerosol flow is thus provided as a side flow to the ejector. The ionized clean gas charges the particles. The charged particles may be further conducted back to the duct or space containing the aerosol. The fine particles of the aerosol sample are thus monitored by monitoring the electrical charge carried by the electrically charged particles. Free ions may further be removed using an ion trap.
A major problem in any particle measurement device which is based on electrically charging particles and measuring the electrical current carried by such charged particles is the conversion of the measured electrical current to actual characteristics of the particle flow, such as particle count, surface area or mass concentration. The conversion factor may be simply determined by calibrating a particle measurement device against a reference method. For example mass concentration is typically calibrated against gravimetric method, which accurately determines mass concentration. However, such calibration may change due to changes in the shape of particle size distribution curve, mean particle diameter, width of a lognormal particle size distribution curve, particle shape (usually expresses with fractal parameters) or particle density.
U.S. Pat. No. 7,812,306 B2, TSI, Incorporated, Oct. 12, 2010, describes an instrument for non-invasively measuring nanoparticle exposure includes a corona discharge element generating ions to effect unipolar diffusion charging of an aerosol, followed by an ion trap for removing excess ions and a portion of the charged particles with electrical mobilities above a threshold. Downstream, an electrically conductive HEPA filter or other collecting element accumulates the charged particles and provides the resultant current to an electrometer amplifier. The instrument is tunable to alter the electrometer amplifier output toward closer correspondence with a selected function describing particle behavior, e.g. nanoparticle deposition in a selected region of the respiratory system. Tuning entails adjusting voltages applied to one or more of the ion trap, the corona discharge element and the collecting element. Alternatively, tuning involves adjusting the aerosol flow rate, either directly or in comparison to the flow rate of a gas conducting the ions toward merger with the aerosol. The publication is focused on the measurement of particle concentrations in terms of surface area, as such accumulated or aggregate surface area are expected to provide more useful assessments of health risks due to nanoparticle exposure. The publication actually teaches that mass concentration measurements are not useful in indicating health effects and thus would not motivate person seeking for a solution on converting measured electrical current into mass concentration to examine the technique described in the publication.
U.S. Pat. No. 8,122,711 B2, Robert Bosch GmbH, Feb. 28, 2012, concerns a procedure to ascertain a concentration of sooty particles in an exhaust gas system of an internal combustion engine or a depletion of an emission control system of the internal combustion engine due to the loading of sooty particles, whereby the sooty particle concentration in the exhaust gas system is determined by means of a collecting particle sensor, which emits a sensor signal and whereby the depletion of the emission control system due to the loading of sooty particles is determined from the sooty particle concentration. The sensor signal is corrected by means of predetermined corrections with regard to a sensor temperature and/or an exhaust gas temperature and/or a flow velocity of the exhaust gas and/or a voltage applied at the particle sensor. Transverse sensibilities of the particle sensor can thereby be taken into account during the evaluation; and the determination of the accumulated loading of sooty particles and the determination of the sooty particle concentration in the exhaust gas system are improved. In the process, the sensor temperature enters into the correction to the extent that a temperature dependence of the electrical resistance of the loading of sooty particles is determined in a preparation phase and can be taken into account during the evaluation of the sensor signal. Although the procedure improves the mass concentration measurement, it involves extra components and is thus clumsy and costly.
Publication WO 2013/132154 A1 presents a prior art solution, where a stable elevated cut-off size of measured particles is utilized to improve the accuracy of the results when indicated as mass concentration. This kind of solution can give good results, if the size distribution of measured aerosol particles is situated in limited and known size range. For optimal results, the setting of the cut-off size should be pre-set to the optimal value for each measurement based on the assumed size range. Another limitation with this kind of solution is that it cannot improve the number-concentration value of measured result.
Thus the particle sensors of the prior art possess the technical problem of the electrical current signal vs. characteristics of particles in the particle flow being sensitive to external conditions. There is need for a sensor which can measure or monitor particle characteristics even when the particle mean diameter is changing. Especially advantageous would be the improvement in the accuracy of both number and mass concentrations without presumptions of the particle size range.